The invention concerns a method of reducing CO and NOx emissions in a heating appliance, and a respective appliance.
Particularly, the invention may be used in heating appliances which are not provided with a flue, for instance stoves and fireplaces fed with a gaseous fuel, which discharge the combustion products in the room where they are installed.
The above-mentioned heating appliances are commonly used to heat rooms which are not provided with a flue to convey the combustion products to the outside.
Since the combustion products are discharged in the room where the appliance is installed, there are very restrictive safety rules concerning the allowable content of toxic substances, such as carbon oxide CO and nitrogen oxides, in said combustion products.
The formation of CO in the combustion products depends on a plurality of factors:
non-complete combustion of the fuel, which may be caused by an insufficient quantity of air in the air-fuel mixture supplied to the appliance;
cooling of flame during combustion, which may be caused by a contact of the flame with cold surfaces having a temperature less than the temperature of the flame, or by a flow of cold air, the so-called secondary air, from the room onto the surface of the flame;
excess of air in the air-fuel mixture supplied to the appliance, which causes a combustion with a too low temperature of the flame, resulting in a non-complete combustion of the fuel.
It is know that the formation of NO2 does not take place directly at the inside of the flame during combustion, but subsequently in a region directly surrounding the flame where NO generated during combustion is oxidized.
Therefore, the formation of NO2 depends both on the presence of NO in the combustion products and on the presence of oxidizers, such as O2 and OH radicals, for instance HO2 radicals, in said region directly surrounding the flame. The quantity of HO2 radical depends on the temperature of the flame and decreases as the temperature increases.
The quantity of NO2 in the combustion products may be reduced to very low values by using a bladed flame burner, such as the burner disclosed in EP-B-0373157 and EP-A-0537244, both in the name of the applicant. This burner makes possible to obtain very low contents of NO2 in the combustion products. However, the use of the above mentioned burner does not allow to reduce CO emissions to very low values.
If a conventional burner with a xe2x80x9cBunsenxe2x80x9d flame is used, it is possible to obtain a low content of CO in the combustion products, but the content of NO2 continues to be high.
EP-A-0512801 discloses a combustion method for a radiant burner and a respective burner fed with a hyperstoichiometric mixture of gas fuels and air. Said method and burner allow the formation of CO and NO2 in the combustion products to be eliminated or, at least, reduced to very low levels because the combustion of said mixture takes place on the surface of the diffuser of the burner, or close to said surface, thus limiting the contact of the combustion products with the ambient air, the so-called secondary air.
The above mentioned method, however, is not able to reduce the formation of CO to acceptable levels in bladed flame burners as described in the above mentioned EP-B-0373157 and EP-A-0537244 and the formation of NO2 in xe2x80x9cBunsenxe2x80x9d flame burners, because the flames of both said burners has a substantial height and the combustion does not take place on the surface of the burner or close to said surface.
In addition, the above method can not be used in a fireplace which is provided with a large front aperture through which the ambient air is free to enter the combustion chamber.
It is also known from EP-A-766046, which is considered the closest prior art, a flueless heating appliance comprising a combustion chamber having an opening in at least one side thereof to provide e view of at least a substantial portion of the combustion chamber, a flammable fluid fuel supply to said combustion chamber to provide, upon combustion, a substantial portion of yellow flame, an artificial log assembly provided in said combustion chamber and at least one reflective surface provided on at least one side of said primary combustion chamber to provide a reflected view of said substantially yellow flame and artificial log assembly. The heating appliance further comprises air circulating means to provide and/or direct a stream of air across at least a portion of said opening into said combustion chamber.
The primary object of the heating appliance disclosed in EP-A-766046 is to provide a realistic visual appearance of a wood fireplace. This object is obtained by generating flames having a substantially yellow appearance in the combustion chamber and by providing the combustion chamber with mirrors which provide a larger visual appearance of the fire. It is known that yellow flames produces combustion products containing a substantial amount of CO and NOx, which can not be exhausted in the environment where the appliance is placed. For this reason, the heating appliance disclosed in EP-A-766046 is provided with a catalytic converter forming a secondary combustion chamber in which the combustion products are further combusted to reduce the content of CO and NOx to acceptable levels. In addition, the air circulating means provided in the heating appliance serve to prevent or at least reduce spillage of the combustion products from the combustion chamber towards the environment in which the heating appliance is placed. The need for a catalytic converter and air circulating means makes the heating appliance disclosed in EP-A-766046 very costly and not completely reliable do to possible faults of the catalytic converter or air circulating means.
It is an object of the present invention to provide a method of reducing CO and NO2 emissions in heating appliances without flue, provided with xe2x80x9cBunsenxe2x80x9d flame burners or bladed flame burners, such as disclosed in EP-B-0373157 and EP-A-0537244. A further object of the present invention is to provide a heating appliance with low emissions of CO and NO2. Such a method and heating appliance should be reliable and not very costly.
According to a first aspect of the present invention there is provided a method of combustion of a gaseous fuel in a heating appliance provided with at least one burner, said method comprising:
mixing said gaseous fuel with a preestablished quantity of air to obtain an air-fuel mixture;
supplying said mixture to said at least one burner;
causing said mixture to pass through openings provided in a diffuser of said at least one burner;
generating flames causing the combustion of said mixture; and irradiating heat towards said flames and a region directly surrounding said flames.
According to a second aspect of the present invention there is provided a method of combustion of a gaseous fuel in a heating appliance provided with at least one burner, said method comprising:
mixing said gaseous fuel with a preestablished quantity of air to obtain an air-fuel mixture,
supplying said mixture to said at least one burner;
causing said mixture to pass through openings provided in a diffuser of said at least one burner;
generating flames causing the combustion of said mixture; and
limiting the inflow of air from the outside of said appliance into a region directly surrounding said flames, said limiting being obtained through gaseous barrier means, wherein said gaseous barrier means is obtained through natural circulation of gaseous means.
According to a third aspect of the present invention there is provided a heating appliance comprising at least one burner fed with a mixture of gaseous fuel and air, said at least one burner being provided with a diffuser having openings through which said mixture passes and with means to cause the combustion of said mixture and the formation of at least one flame, wherein said heating appliance further comprises heat irradiating means for irradiating heat towards said at least one flame and a region directly surrounding said flame.
According to a fourth aspect of the present invention there is provided a heating appliance comprising at least one burner fed with a mixture of gaseous fuel and air, said at least one burner being provided with a diffuser having openings through which said mixture passes and with means to cause the combustion of said mixture and the formation of flames, said heating appliance comprising barrier means capable of limiting the inflow of air from the outside into said heating appliance in a region directly surrounding said flames, said barrier means comprising gaseous barrier means, wherein said gaseous barrier means is obtained through natural circulation of gaseous means.
Irradiating heat on the flame and in the region directly surrounding the flame makes it possible to keep in said region a temperature high enough to substantially reduce the formation, and the presence, of CO and NO2 in the combustion products.
In addition, the provision of gaseous barrier means to limit the inflow of air from the outside into the heating appliance minimizes losses of heat from the combustion chamber, preventing the temperature in the region of the flames from decreasing below the value required for minimizing the formation of CO and NOx.